a)  

cos a= -5/13

pi/2 < a < pi

sin a = √ 1-cos^2 = √ 1-(-5/13)^2 = 12/13

tg a = sin /cos = -5/13  / 12/13 = -5/12

ctg a = 1 / tg a = 1/ -5/12 = - 2,4

a)

sin a = -0.8

3pi/2 < a < 2pi

cos a = √ 1-sin^2 = √ 1- (-0.8)^2 = 0.6

tg a = sin /cos = -0.8/0.6 = - 4/3

ctg a = 1 / tg a = 1/ - 4/3 = -3/4

b)

sin a = √3/3

pi/2 < a < pi

cos a = √ 1-sin^2 = √ 1- (√3/3)^2 = -√(2/3)

tg a = sin /cos = √3/3 / -√(2/3) = -1/√2 = -√2/2

ctg a = 1 / tg a = 1 / -1/√2 = -√2

 b)

cos a= 1/7

0 < a < pi/2

sin a = √ 1-cos^2 = √ 1- (1/7)^2 = 4√3/7

tg a = sin /cos = 4√3/7  /  1/7 = 4√3

ctg a = 1 / tg a = 1/  4√3 = √3/12